Studies on isolated cell components

1. The addition of heparin to rat liver, kidney, or brain nuclei has been found to bring about the release of a gel. Chemical analysis and histochemical studies on whole homogenates and isolated nuclei demonstrated that the material released by heparin contained desoxyribonucleic acid (DNA) and protein. The action of heparin on nuclei is interpreted as the result of a combination with the basic proteins of the nucleus with a consequent displacement of DNA. 2. The addition of heparin to a finely divided dilute liver homogenate prepared in a phosphate-sucrose solution at pH 7.1 brings about a marked increase in viscosity which reaches a maximum in 6 to 8 minutes at 23° and then declines. 3. The concentration threshold for the viscosity effect was 0.1 mg. per 100 mg. fresh rat liver, with further increases in viscosity at higher heparin concentrations. Over a period of several hours a marked decrease in response to heparin was observed in homogenates stored at 0°. 4. Fractionation of the homogenate demonstrated that the viscosity increase was due to the presence of the nuclei alone, other components showing no effect. Microscopic observation showed that the increase in viscosity was associated with the appearance of a clear gel around nuclei treated with heparin. 5. Heparin brought about the release of DNA from the nuclei of incubated rat liver, kidney, and brain homogenates. In some instances over half the DNA is found in the supernatant after high speed centrifugation (20 minutes, 21,000 x g). 6. No correlation was found between anticoagulant activity of heparin preparations and their effectiveness in causing an increase in the viscosity of liver homogenates. Desulfated heparin produced none of the results described here for heparin.     

Studies on isolated cell components. IV. The effect of various solutions on the isolated rat liver nucleus

1. The effects of morganic ions, electrolyte concentration, and pH on the appearance and volume of the isolated rat liver nucleus have been studied. Nuclei were isolated by differential centrifugation in a buffered salt-sucrose mixture at pH 7.1. Nuclear volumes were determined photographically. 2. In solutions of NaCl, of KCl, and in potassium phosphate buffers the nuclear volume decreased markedly with an increase in concentration from 0.001 M to 0.05 M but remained essentially constant with further increase in concentration to 1.0 M. The effects of CaCl(2) and MgCl(2) differed from those of NaCl and KCl in that a smaller volume was obtained in concentrations less than 0.15 M, and in the case of CaCl(2) an increase in volume was obtained in more concentrated solutions. The volume changes are considered to be due primarily to ionic effects on the nuclear colloids rather than to osmotic behavior. 3. Treatment of nuclei with DNAase prevented the characteristic volume changes resulting from ion effects, suggesting the importance of DNA in nuclear volume changes. 4. The optical changes in isolated nuclei in various concentrations of KCl, NaCl, CaCl(2), MgCl(2), and in potassium phosphate buffers as observed under phase contrast illumination are described. CaCl(2) gave the most marked nuclear changes from the conditions in the uninjured cell and caused shrinkage and granulation in 0.001 M concentration. The effects of CaCl(2) were also manifested in 0.88 M sucrose, in mixtures with monovalent salts, and in serum. Changes in nuclear volume and optical appearance which occurred in salt solutions and in 0.1 N HCl were readily reversible. 5. Nuclear volume remained constant between pH 8.91 and 5.12 and decreased in more acid solutions. 6. Sucrose had no appreciable osmotic effect, and in hyperosmotic solution. (0.88 M) nuclei showed swelling and rupture comparable to that in distilled water. 7. The results are considered in relation to the requirements of nuclear isolation media. 8. Rat liver nuclei isolated in a buffered salt-sucrose medium by differential centrifugation exhibited a pattern of size distribution similar to that of fixed nuclei but were of considerably larger volume. The ratio of the volumes of the peak frequencies of the two chief size groups was 1:1.9.     

Studies on isolated cell components: I. Nuclear isolation by differential centrifugation

A method is presented for isolating nuclei of rat and hamster liver in a high state of purity and in a condition optically similar to nuclei within living cells.The isolation procedure consists in the homogenization and differential centrifugation of fresh liver at 0–5 ° in a salt-sucrose solution buffered at pH 7.1. By layering the material to be centrifuged over a relatively large volume of a slightly denser solution the purification can be carried out in 4 or 5 centrifugations. The entire procedure can be completed in about 90 minutes. The yield as determined by measurements of desoxyribonucleic acid is about 5 per cent.The solutions contain KH₂PO₄, K₂HPO₄, NaHCO₃, and sucrose. The sucrose concentration is varied to give density differences required for layering. Salt-sucrose solutions maintain a large proportion of the isolated nuclei in a nongranular condition for 6 hours at 0 °. Pure sucrose is less satisfactory for maintenance.The protein-desoxyribonucleic acid ratio for the isolated nuclei averages 5.1 with a range of 2.7 to 8.9.     

Influence of glycosaminoglycans on endogenous DNA synthesis in isolated normal and cancer cell nuclei. Differential effect of heparin

The influence of exogenously-added glycosaminoglycans and glycoproteins on DNA synthesis in isolated nuclei, from normal and malignant tissues, was investigated. Heparin stimulated DNA synthesis in normal cell nuclei at concentrations (heparin/DNA (w/w) <0.9) which inhibited DNA synthesis in tumor cell nuclei. At higher concentrations (heparin/DNA (w/w) > 0.9) heparin inhibited DNA synthesis in both normal and tumor cell nuclei. The chondroitin-4 and 6-sulfates, heparan sulfate, cartilage proteoglycan, N-desulfated heparin, and glycophorin caused inhibition of DNA synthesis at all concentrations tested and in all nuclei examined. Hyaluronic acid, dermatan sulfate, keratan sulfate, α₁-acid glycoprotein and fetuin had no significant influence on DNA synthesis in isolated nuclei.     

Action of heparin on protein fractions of isolated nuclei and on their DNA content.

Isolated nuclei of rat hepatocytes were incubated with 0.05% sodium heparinate for 2 to 10 min. Alterations in the nuclei were controlled biochemically, determining the amounts of DNA and histones, and by cytophotometric methods determining the amounts of total and nonhistone proteins and DNA. Under the selected experimental conditions 95% of histones are bound already after 5-min incubation with heparin; nonhistone proteins of cell nuclei remain unchanged. The blockade of histones is followed by DNA diffusion into the incubation medium. Experiments with nuclear staining with alcian blue proved the specificity of heparin binding with histones and showed that heparin-histone complex remains in the nuclei, and its histones lose their extractability with 0.25 n HCl.     

A comparative study of the protein components of ribonucleoprotein particles isolated from rat liver and hepatoma nuclei

To solve the mechanism for the complete cessation of DNA synthesis in Tetrahymena cells involved in the amino acid starvation, the nature of DNA polymerase activity was investigated in crude enzyme preparations or in toluene-permeabilized specimens. In crude enzyme preparations from growing cells, ³H-TTP incorporation into acid-insoluble products showed little dependency on exogenous DNA template, while incorporation increased markedly in the presence of ATP. These characteristics were very similar to those of replicative DNA synthesis in permeabilized Escherichia coli.Variations of DNA and RNA polymerase activities following transfer of exponentially growing Tetrahymena cells to amino acid-deprived medium showed that in the crude enzyme preparations DNA polymerase activity dropped sharply within 3 h after the transfer and practically no activity was detected thereafter, whereas RNA polymerase activity did not disappear in the same preparations. Such enzyme kinetics coincided well with the kinetics of in vivo synthesis of the corresponding nucleic acid.The cessation of DNA synthesis in the amino acid-starved cells may be due not to the activation of DNase or a soluble polymerase inhibitor, nor to the deficiency of each kind of deoxyribonucleoside triphosphate or magnesium ion or ATP generation system. It follows from this that the cessation of DNA polymerase activity in the starved cells may be due to the deficiency of DNA polymerase or its associated factor(s) as a reflection of short life-span of such a protein.     

Studies on the mechanism of the rate-enhancing effect of heparin on the thrombin-antithrombin III reaction.

The rate of the reaction between thrombin and antithrombin III is greatly increased in the presence of heparin. Several mechanisms for this effect are possible. To study the problems commercial heparin was fractionated into one fraction of high anticogulant activity and one of low anticoagulant activity by affinity chromatography on matrix-bound antithrombin III. The strength of the binding of the two heparin fractions to antithrombin III and thrombin, respectively, was determined by a crossed immunoelectrophoresis technique. As was to be expected, the high activity fraction was strongly bound to antithrombin III while the low activity fraction was weakly bound. In contrast, thrombin showed equal binding affinity for both heparin fractions. The ability of the two heparin fractions to catalyse the inhibition of thrombin by antithrombin III was determined and was found to be much greater for the high activity heparin fraction. A mechanism for the reaction between thrombin and antithrombin III in the presence of small amounts of heparin is suggested, whereby antithrombin III first binds heparin and this complex then inhibits thrombin by interaction with both the bound heparin and the antithrombin III.     

RNA synthesis in isolated HeLa cell nuclei.

RNA synthesis has been studied in isolated nuclei of HeLa cells. The incubation medium has been optimized for RNA synthesis and the requirements for the presence of specific components previously used by other investigators has been examined. Nuclei isolated by centrifugation through 2 M sucrose synthesize RNA linearly for at least 1 h only at low temperature (25 degrees C). Low molecular weight RNA is found in the supernatant fraction after incubation; this RNA accounts for about 10% of the RNA synthesized. The RNA which remains within nuclei is of high molecular weight and processing of this RNA into molecules of the size of cytoplasmic mRNA does not seem to occur in isolated nuclei. We have studied the effect of an inhibitor of protein-nucleic acid interaction - aurintricarboxylic acid - on RNA synthesis by isolated nuclei. At concentrations below 0.1 mM, this drug does not inhibit RNA synthesis effectively, whereas at concentrations above 0.1 mM it inhibits RNA synthesis by about 80%. In view of the proposed mechanism of action of aurintricarboxylic acid, we suggest that completion of nucleotide chains initiated before nuclei isolation accounts for 20% of the RNA synthesized in our system by isolated nuclei, whereas nucleotide chains initiated during the in vitro incubation account for 80% of the RNA synthesized.